Magnetic recording media such as a magnetic tape are generally manufactured by a method of coating a magnetic paint comprising magnetic powder, binder, and resin on a base material, then drying it.
Recently, in the field of magnetic recording medium, especially in the field of magnetic medium for videorecording which is required to record a high frequency signal, a recording medium with a plurality of magnetic layers is proposed for the purpose of increasing magnetic recording capacity, and improving as well as balancing magnetic recording characteristics in both the high frequency and low frequency area. The examples are shown in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O. P. I. Publication) Nos. 98803/1973, 172142/1984, Japanese Patent Examined Publication No. 2218/1957, Japanese Patent O. P. I. Publication No. 64901/1976, Japanese Patent Examined Publication No. 12937/1981.
However, the conventional techniques have not clearly disclosed the conditions to form an upper magnetic layer with good surface conditions on a lower magnetic layer, or the mutual relation of coating conditions of each layer in order to form magnetic layers with adequate surface conditions so that high output and a high S/N ratio can be obtained. Accordingly, some problems are caused as follows. The surface of a magnetic recording medium becomes too smooth like a mirror surface in some cases, so the rubbing noise of the tape in its runing is too loud. In some cases the surface of a magnetic recording medium is so rough that C/N ratio, a ratio of carrier to noise, is lowered.
In Japanese Patent O. P. I. Publication No. 66228/1983, on the other hand, it is disclosed that the reproducing output can be increased over all frequency bands from the low frequency band to the high frequency band by using of a magnetic powder having a BET value of 28 m.sup.2 /g to 45 m.sup.2 /g, and that having a BET value of 18 m.sup.2 /g to 25 m.sup.2 /g in the upper and lower magnetic layer respectively. An example is disclosed in which the same binder is used in each of the upper and lower layers. When the same binder is used in each of the upper and lower layers, aggregation rarely occurs on the boundary between the two layers and affinity between the two layers is improved. As a result, the surface of the upper layer is improved.
Generally, the upper layer is thinner than the lower layer. The upper layer thickness is generally 0.6 .mu.m to 1 .mu.m and the lower layer thickness is 1.5 .mu.m to 4 .mu.m. The finished surface of the upper layer is subject to the surface conditions of the lower layer, therefore the upper layer surface tends to become rough, being affected by the lower layer. But in the case of a magnetic medium disclosed here. BET value of the magnetic powder in the upper layer is comparatively large and the surface area of the particles is large or, in other words, the particle size is small. Therefore, the surface of the upper layer becomes flat and smooth. In addition to that, the surface of the upper layer is further improved because the same binder is used in each of the upper and lower layers.
But in the case of the magnetic medium disclosed in the above-mentioned Japanese Patent O. P. I. Publication No. 66228/1983, abrasive is not contained in the magnetic layers, especially in the upper layer. For that reason, the surface conditions of the upper layer is improved. On the other hand, abrasive efficiency of the magnetic layer is low and the surface becomes smooth beyond necessity. For the reasons explained above, the rubbing noise became louder when the surface of the tape came into sliding contact with a magnetic recording head. Furthermore, it was proved that RF output, still frame durability and the electro-magnetic conversion characteristic of the medium were degraded. It was also proved that formation of chips of magnetic powder fallen at the time of slitting of the medium were increased, and these chips could be adhered to the surface of the medium to cause dropout. It is thought that the magnetic powder falling was caused by large BET value of the upper layer magnetic powder.
A magnetic medium which contains abrasive in both the upper magnetic layer and the lower one, is shown in Japanese Patent O. P. I. Publications No. 146211/1988 and No. 47228/1985. The medium have disadvantages as follows. The patent publications describe that 1 or 2 parts by weight of alumina per 100 parts by weight of magnetic powder is added into the upper layer and the lower layer of the medium. But it was proved that those were not sufficient to reduce the rubbing noise and to raise still frame durability and RF output of the medium. It is thought that the cause is not only the amount of aluminum but also the very thin magnetic upper layer of the medium. In other words, it is speculated that there exist particular problems which can not be predicted in case of recording medium having a thicker single magnetic layer. Any measure to solve such problems is not described in the above-mentioned publications at all.